In storage phosphor radiography systems, a storage phosphor is exposed to radiation, to produce a latent image in the storage phosphor. Subsequently, the storage phosphor is scan-simulated to release the latent image in the form of detectable radiation. One of the problems associated with storage phosphor radiography is to determine the intensity of stimulation required to produce an optimum read-out of the storage phosphor. The optimum stimulation intensity depends upon the range of energies stored in the phosphor. To this end, it has been proposed to conduct a preliminary scan-stimulation at low intensity to determine the range of energies stored in the storage phosphor. A final read-out scan-stimulation is then performed based on the results obtained from the preliminary read-out. The intensity of the final read-out scan is also referred to as final scan "gain". See for example European Patent application EP 00778677 A3, published Apr. 27, 1983 Suzuki and Horikawa, where they suggest the use of a preliminary scan as a means of determining the final scan and image processing conditions. Although they proposed storage phosphor radiography systems employing manual and automatic control units that utilize the pre-scan information to determine the final scan conditions, they did not disclose the details of how these units work.
In U.S. Pat. No. 4,682,029 issued Jul. 21, 1987 to Tanaka et al., they used the pre-scan image histogram to determine the minimum (S.sub.min), and the maximum (S.sub.max) signal levels that correspond to the "useful" image information. The final scan gain was determined such that (S.sub.min) and (S.sub.max) would become respectively the the signal levels Q.sub.min and Q.sub.max. At the output, predetermined transformation mapped the signal within the range [Q.sub.min, Q.sub.max ] to the desired output density range [D.sub.min, D.sub.max ]. In this manner, the useful image information was expressed within a predetermined range at the output. This technique used a "percent rule" to determine S.sub.min and S.sub.max from the pre-scan image histogram. The quantity S.sub.max was determined from a gray level that was occupied by 0.1 to 2.0% of the total number of picture elements and S.sub.min was determined from a gray level that was occupied by 0.05 to 1.0% of the total number of picture elements. The major drawback of this technique is that many gray levels may have the same relative percent population. No rule was disclosed to choose among the multiple possibilities.
In European patent application EP 0145982 A1, published Jun. 26, 1985, Tanaka et al. used a slightly different but equivalent perspective in considering the problem. They emphasized the automatic control of a scale factor introduced in the analog-to-digital (A/D) converter. That is, the final scan gain and the scaling, followed by A/D conversion, constituted the final scan conditions. As in U.S. Pat. No. 4,682,028 cited above, the pre-scan histogram was used to determine S.sub.min and S.sub.max. The scale factor was determined from the difference (S.sub.max -S.sub.min). In cases where the spatial extent of the radiation exposure field is limited to a certain anatomical structure (i.e., collimated X-rays), the value S.sub.min is determined mainly by the scattered radiation. This value is smaller than that obtained within the image portion of the radiation exposure field. As a result, the image contrast may decrease if this fact is not taken into consideration in determining the scale factor. In order to alleviate such detrimental effects, Tanaka et al. proposed a technique that required the computation of the histogram (h.sub.2) of the pre-scan data obtained from a sub-region of the storage phosphor plate in addition to the histogram (h.sub.1) of the data obtained from the entire storage phosphor plate (sub-region area normally occupied 20% to 80% of the total plate area). The quantities S.sub.min,1, S.sub.max,1 and S.sub.min,2, S.sub.max,2 were obtained from h.sub.1 and h.sub.2, respectively. (Usually, S.sub.min,1 &lt;S.sub.min,2 and S.sub.max,1 =S.sub.max,2 =S.sub.max.) Tanaka et al. proposed a method to compute a value of S.sub.min from S.sub.min,1 and S.sub.min,2, that would be used to determine the scale factor.
In European patent application EP 0154880 A2, published Sep. 18, 1985 by Tanaka et al., the pre-scan data were collected only from selected sub-regions of the phosphor plate. A characteristic value, S.sub.ch, was calculated from the mean values of the gray levels within these sub-regions. The final scan gain was determined such that S.sub.ch would become the gray level Q.sub.av at the final scan. In the predetermined output transformation, the quantity Q.sub.av was mapped to a desirable output density level D.sub.av. The major disadvantage of this technique is that the location of the sub-regions that are used to collect pre-scan data are exam-, and possibly image-dependent.
In U.S. Pat. No. 4,652,999 issued Mar. 24, 1987, Higashi et al. proposed a configuration where the final scan gain and the image processing conditions were determined automatically from the pre-scan information. The so-called "automatic sensitivity adjusting function" (ASAF) determined the final scan gain based on the exam type and image recording conditions (e.g., chest exam and lung field magnification). The final scan gain was determined such that the image information presented to the output station was within a predetermined range [Q.sub.min, Q.sub.max ], which was mapped to some predetermined density range [D.sub.min, D.sub.max ] at the output. But, a desired D.sub.max may have been specified for the lung field only rather than for the entire image. In that case, the lung field can have the desired output dynamic range only if the x-rays were coned (collimated) onto the lung field (lung field magnification image). Higashi et al. addressed this problem by proposing a "secondary automatic gradation" unit which would ensure that the structure of interest, rather than the entire image, had the desired dynamic range for varying image recording conditions. This control unit was provided with the recording conditions and the value at the output of the ASAF unit. The working principles of the ASAF unit for determining the final scan gain were not disclosed.
As further experience with storage phosphor imaging systems has been gained, it has become apparent that further improvements in methods for adjusting the final read-out conditions based on a preliminary read-out are needed.
Another problem that has been discovered as experience has been gained is that unsatisfactory exposures are not discovered until the final image is read out, processed and displayed. This whole process can consume a good deal of computer time that is wasted if the image must be re-taken.